Numerical control grinding machine

ABSTRACT

In a numerical control grinding machine using a grinding wheel made of cubic boron nitride, a computerized numerical controller controls the infeed movement of a wheel head to effect a rough grinding, a drifty grinding and a fine grinding on a rotating cylindrical workpiece. The drifty grinding due to a spring-back motion which is caused by a flexed workpiece is initiated in response to a sizing signal from a sizing device which measures the workpiece diameter varying momentarily during the grinding operation and is terminated upon the expiration of a predetermined time which is sufficient to vanish the variation of the workpiece diameter. A sizing device measures the actual diameter thereafter, and the sizing signal is compensated in order to approximate the actual diameter to a desired set size which is the sum of a finish diameter and a predetermined allowance for the subsequent fine grinding operation, as the number of the workpieces increases. That is, the numerical controller diminishes a set size which determines the time point to issue the sizing signal from the sizing device, toward the desired set size on a step-by-step basis with the increase in number of the workpieces ground.

BACKGROUND OF THE INVENTION

1. Field of the Invention

The present invention relates to a grinding machine wherein in responseto a signal issued from a sizing device when a workpiece being groundreaches a predetermined size, the infeed movement of a grinding wheelcarrier is halted prior to a fine grinding operation for removing theflexing of the workpiece.

2. Discussion of the Prior Art

Up to now, there is known a numerical control grinding machine whereinthe grinding mode is changed in response to a first sizing signal issuedwhen a workpiece is ground to a predetermined size. However, theworkpiece is ground by a grinding wheel at a predetermined infeed rateso that the workpiece is flexed toward the feed direction of thegrinding wheel. Where the infeed movement of the grinding wheel ishalted upon detecting the first sizing signal, a problem arises in thatthe workpiece is continued to be further ground an amount due to itsspring-back motion which is caused by the flexed workpiece. Hereafter,the grinding caused by the spring-back motion will be referred to as"drifty grinding", and similarly, the grinding amount in such "driftygrinding" will be referred to as "drifty grinding amount". Therefore, itis necessary to add the drifty grinding amount to a set size whichdetermines the time point to issue a second sizing signal for theinitiation of a fine grinding operation.

By the way, in the case of CBN (Cubic Boron Nitride), because thegrinding capability is small right after each truing operation as wellas at the first use of new grinding wheel, the drifty grinding amountincreases. Further, as the number of the ground workpieces increases,the drifty grinding amount decreases by reason that the cutting edge ofeach abrasive grain is broken so that the grinding capability becomeslarge. Therefore, if the set size of the workpiece diameter uponissuance of the first sizing signal is constant without regard to thenumber of ground workpieces, the grinding accuracy may be degraded withincrease in the number of workpieces, and in addition, the grindingcycle time and the feed amount of a rest jaw may be varied.

In order to solve such problems, in the prior art, the set size of theworkpiece diameter upon which the first sizing signal is issued isdiminished with increase in number of ground workpieces based on theexperimental relation between the number of ground workpieces and thecutting quality of the CBN grinding wheel. As a consequence, the driftygrinding amount is controlled to be nearly constant throughout all thegrindings of workpieces.

However, the relation between the number of ground workpieces and thecutting quality varies with the kind of CBN grinding wheel used, thematerial of workpieces, the grinding amount par workpiece and so forth.For this reason, in the prior art method wherein the set size whichdetermines the time point to issue the first sizing signal is altered apredetermined value each time a workpiece is ground, there is raisedanother problem that the relation between the set size above and thenumber of ground workpieces has to be altered to meet respectiveconditions such as the kind of grinding wheels, the material ofworkpieces, the grinding amount par workpiece and so forth.

SUMMARY OF THE INVENTION

It is therefore a primary object of the present invention to improve thegrinding accuracy of the workpiece without relation to the kind ofgrinding wheels, the material of workpieces and the grinding amount parworkpiece by automatically altering to a proper value a set size whichdetermines the time point to issue a first sizing signal.

Another object of the present invention is to provide a high efficiencynumerical control grinding machine wherein an allowance for a finegrinding can be maintained constant notwithstanding that the cuttingquality of a grinding wheel used varies largely with the increase innumber of the workpieces ground after each truing operation.

Briefly, in a numerical control grinding machine according to thepresent invention, a sizing signal is issued from a sizing device whenthe actual diameter of a workpiece observed thereby coincides with a setsize stored in a data storage device, and the infeed movement of thegrinding wheel is halted upon issuance of the sizing signal. Thereafter,the actual diameter is measured by the sizing device in response to adiameter read command generated upon the satisfaction of a predeterminedcondition, such as, for example, a sufficient time which requires tomake the aforementioned drifty grinding completed or state in which thevariation velocity of workpiece diameter reaches a tolerable range. Acalculation is then performed for the difference value between themeasured diameter above and a reference size which is determined takinginto account an allowance left after the drifty grinding. The set sizewhich determines the time point to issue the sizing signal iscompensated for the difference value. Thus, as the number of groundworkpieces increases, the size of the workpiece diameter upon completionof the drifty grinding after the halt of the infeed movement approachthe reference size. Consequently, the accuracy of the ground workpiecescan be improved and the allowance for a subsequent fine grindingoperation becomes constant, because the sizing signal is issued when anobserved diameter coincides with the diameter having been compensatedfor the difference value which has been calculated in the grinding ofthe preceding workpiece.

BRIEF DESCRIPTION OF THE ACCOMPANYING DRAWINGS

The foregoing and other objects, features and many of the attendantadvantages of the present invention will be readily appreciated as thesame becomes better understood by reference to the following detaileddescription of preferred embodiments when considered in connection withthe accompanying drawings, wherein like reference numerals designateidentical or corresponding parts throughout the several views, and inwhich:

FIG. 1 is a schematic view showing the structure of a numerical controlgrinding machine according to the present invention;

FIG. 2 is a block diagram showing electrical components of a numericalcontroller;

FIG. 3 is a flow chart of a subroutine executed by a central processingunit in a first embodiment of the present invention;

FIG. 4 is a graph explaining the state that a set workpiece diameterwhich determines the time point to issue a first sizing signal from asizing device is varied toward a desired value; and

FIG. 5 is a flow chart of another subroutine executed by the centralprocessing unit in a second embodiment of the present invention.

DETAILED DESCRIPTION OF PREFERRED EMBODIMENTS

Preferred embodiments of the present invention will be described withreference to the accompanying drawings.

A work head 12 rotatably carrying a work spindle 13, which is driven bya servo motor 14, is guided slidably on a table 11. A tail stock 15 isfastened on the right end of the table 11 and a workpiece W is carriedbetween centers 16 and 17 of the tail stock 15 and a work spindle 13.The workpiece W is formed at its one end with a pin hole, into which alocating pin 18 protruding from the work spindle 13 is snugly inserted,so that the angular phase of the workpiece W coincides with that of thework sprindle 13.

At the rear top of the bed 10, the wheel head 20 is drivingly connectedwith a servomotor 23 through a feed screw (not shown) and is advanced orretracted in a direction perpendicular to the workpiece axis by rotatingthe servomotor 23 forward or backward.

On the other hand, a steady rest 60 with a jaw 60a which supports theworkpiece W against the grinding wheel G is provided at the front of thebed 10 to be moved in the direction of U axis by a hydraulic actuator(not numbered). The steady rest 60 is disposed at the position whichfaces the grinding wheel G, as disclosed in U.S. Pat. No. 4,711,054 tothe same assignee of this application. A pair of measuring probes 62aadjustable with the distance therebetween protrude from a sizing head62, which is carried on the steady rest 60. The distance between theprobes 62a is adjusted to a finish size of the workpiece diameter by anumerical controller 30 through an NC sizing and rest controller 61, andmoreover, the probe distance so adjusted is determined to indicate azero value point. While the diameter of the workpiece W is observed, theprobe distance varies momentarily with the variation of the workpiecediameter, because the probes 62a are spring-biased to close, as wellknown in the art. More specifically, the sizing head 62 is provided witha differential transformer (not shown) which converts the relativedisplacement between the probes 62a into a corresponding electricsignal, which is input to the NC sizing and rest controller 61 so as tobe converted from analog form into digital form by means of an A-Dconverter (not shown) therein. The digital signal is output form the NCsizing and rest controller 61 to the numerical controller 30. Further,the NC sizing and rest controller 61 is operable to receive a set sizefrom the numerical controller 30 and to issue a first sizing signal tothe numerical controller 30 when the actual diameter observed by theprobes 62 a coincides with the set size, as described later in detail.

Drive units 40, 41 are for respectively driving servomotors 23, 14 inresponse to command pulses from the numerical controller 30. Thenumerical controller 30 mainly controls the servomotors 14, 23 tosynchronize the same with each other and further controls feed movementof the steady rest jaw 60a in order to grind the workpiece W. There areconnected with the numerical controller 30, a tape reader 42 and akeyboard 43 for inputting machining cycle programs, control parametersand various other data, and a CRT display device 44 for displayingvarious information.

As shown in FIG. 2, the numerical controller 30 is mainly composed of amain CPU (central processing unit) 31 for controlling the grindingmachine, a ROM (read-only memory) unit 32 having stored programs tocontrol the grinding machine, a RAM (random access memory) unit 32having stored input data such as a tolerable value regarding thevariation of the workpiece diameter, and an input/output interfacecircuit 34. More specifically, in the RAM unit 32, there are formed asize setting area 320 for storing a set size of the workpiece diameterat which the first sizing signal is to be issued in order to halt infeedmovement of the grinding wheel G, a reference size setting area 321 forstoring a reference size indicative of a theoretical diameter which theworkpiece W would have upon completion of a drifty grinding, and an NCdata area 322 for storing NC data. The numerical controller 30 furtherincludes a drive CPU 36, another RAM unit 35 and a pulse distributioncircuit 37 which constitute a system to drive the servomotors 14, 23.The RAM unit 35 is for storing positioning data of the grinding wheel Ginput from the main CPU 31. The drive CPU 36 performs theslow-up/slow-down control on feed movements of the grinding wheel G, thecalculation for interpolation points up to each objective point and theprocessings for outputting positioning data of the interpolated point ata predetermined time interval. The pulse distribution circuit 37 outputsfeed command pulses.

The machining operation performed by the apparatus as constructed abovewill be described hereafter.

First of all, an initial size A1 of the workpiece diameter upon whichthe first sizing signal is to be issued is set in the size setting area320 of the RAM unit 32. Thereafter, a grinding cycle program shown inFIG. 3 is started when a grinding start command is given. At step 100,the foregoing set size A1 stored in the size setting area 320 is outputto the NC sizing and rest controller 61, so that a time point at whichthe first sizing signal is to be issued is set therein. At next step102, the grinding wheel G is advanced to initiate the rough grinding ofthe workpiece W. Thereafter, at step 104, the main CPU 31 executes aprocessing for ascertaining whether or not, the first sizing signal hasbeen output from the NC sizing and rest controller 61. Step 106 followsupon detecting the sizing signal thereat and the advancement of thewheel head 20 halts.

Subsequently, at next step 108, the wheel head 20 with the grindingwheel G rotating remains being halted for a predetermined time. As aresult, the drifty grinding due to a spring-back motion caused by theflexed workpiece W is performed to remove an amount D1 from theworkpiece W during this waiting time. The predetermined time for thiswaiting is sufficient to make the variation of the workpiece diameter tovanish, namely to return the workpiece axis to an ideal position. Thatis, the first sizing signal is issued when the actual workpiece diametervarying momentarily reaches the set size A1, whereby the infeed movementof the grinding wheel G is halted, during which time the workpiece W issubjected to the drifty grinding due to spring-back motion until it isground to a size B1 which is smaller by a value D1 than the set size A1in diameter, as shown in FIG. 4. And at step 110 (i.e., at theexpiration of the predetermined waiting time), a diameter read commandis input to the NC sizing and rest controller 61, which thus outputsdata indicating the actual diameter of the workpiece W to the numericalcontroller 30. At next step 112, the data (the diameter size B1 uponcompletion of the drifty grinding) is read and stored as a variable X.In the next place, at step 113, the roundness of the ground workpiece Wis improved by advancing the jaw 60a of the steady rest 60 to supportthe workpiece W, and at the same time, the wheel head 20 is advanced toperform a fine grinding operation. At step 114, the wheel head 20 isretracted to complete the fine grinding operation when a second sizingsignal is input from the NC sizing and rest controller 61 to thenumerical controller 30.

It is to be noted that the reference size R which the workpiece W shouldhave upon completion of the drifty grinding has been stored in thereference size setting area 321 in advance.

At next step 116, a difference value F1 between the reference size R andthe diameter size B1 is calculated and stored as a compensation value orvariable Δ. And at step 118, it is ascertained whether or not thevariable Δ is larger than, or equal to, a predetermined value. When thevariable Δ is larger than, or equal to, the predetermined value, step120 follows, wherein the set size A1 stored in the size setting area 320is replaced with a new set size A2 which is obtained by subtracting avalue α (half of the variable Δ) from the present set size A1.

In this manner, the processing for the first workpiece W is finished. Inthe grinding of a second workpiece W, the first sizing signal is issuedwhen the actual workpiece diameter varying momentarily coincides withthe set size A2 and the infeed movement of the wheel head 20 is haltedfor the predetermined time. Similarly, the drifty grinding is thenperformed, and after the fine grinding operation, another differencevalue F2 between the actual workpiece diameter at the end of the driftygrinding and the reference size R is calculated and stored as a newvariable Δ. Further, for the subsequent grinding of a third workpiece W,the workpiece diameter size upon issuance of the first sizing signal iscompensated to coincide with a further new set size A3 through steps 118and 120.

As the number of the ground workpieces increases, the workpiece diameterupon which the drifty grinding is completed gradually approaches thepredetermined reference size R, and the fine grinding amounts of theworkpieces W become constant. As a result, the accuracy of finishworkpieces can be improved and in addition, finish sizes of theworkpieces W can be uniformed.

The second embodiment of the present invention will be described withreference to a flow chart shown in FIG. 5 which indicates the executionsteps of the main CPU 31.

Although in the first embodiment, measuring the workpiece diameter isperformed once upon completion of the drifty grinding, namely, when asufficient time passes to vanish the variation of the workpiece diameterdue to the spring-back motion, the measurement of the workpiece diameterin this second embodiment is repetitively performed at a predeterminedtime interval during the drifty grinding. That is, in the secondembodiment, the fine grinding operation is initiated when the velocityat which the workpiece diameter varies for the predetermined timeinterval reaches a tolerable value or less. Therefore, the RAM unit 32in the second embodiment is further provided with another setting area323 for storing a diameter tolerable variation value E, which will bedescribed later in more detail.

The operation of the second embodiment, performed by the apparatus asconstructed above, will be described hereafter.

Steps 200-206 are equivalent to those steps 100-106 in the firstembodiment, respectively. At step 208, a diameter read command is issuedto the NC sizing and rest controller 61, and at next step 210, theworkpiece diameter is measured and stored as a first variable XO. In thenext place, step 212 is executed to await for a predetermined time T,and a step 214, the next diameter read command is issued, in response towhich at next step 216, the workpiece diameter is measured again to bestored as a second variable X.

And, at step 218, the variation velocity Vv at which the workpiecediameter varies for the predetermined time interval (a limited time) iscalculated using the following expression. ##EQU1##

It is then ascertained whether or not, the variation velocity Vv isequal to, or smaller than, the tolerable value E. If the variationvelocity Vv is larger than the tolerable value E, namely when the driftygrinding substantially continues, step 220 is reached, wherein thesecond variable X is stored as the first variable XO. At next step 222,the routine remains as it is for the predetermined time period and then,returns to step 214 wherein the workpiece diameter read command isissued. Thus, the workpiece diameter changing momentarily is measuredagain by the NC sizing and rest controller 61, and subsequently at step216, the newly measured value thereof is stored as the second variableX.

And at step 218, in the same manner as above, the variation velocity Vvis calculated on the basis of the difference value between X and XO andthe time interval T, and thereat, a processing is further made forascertaining whether or not the variation velocity Vv is small than, orequal to, the tolerable value E. If the variation velocity Vv is smallthan, or equal to the tolerable value E, step 224-232 are executedsimilarly to those in the first embodiment.

From the foregoing, it is apparent that the second embodiment has theadvantage that the grinding cycle time becomes shorter, compared withthe first embodiment which requires the predetermined sufficient time tocomplete the drifty grinding.

Obviously, numerous modifications and variations of the presentinvention are possible in light of the above teachings. It is thereforeto be understood that within the scope of the appended claims, thepresent invention may be practiced otherwise than as specificallydescribed herein.

What is claimed is:
 1. A grinding machine wherein a sizing device isprovided for outputting a sizing signal when a cylindrical workpiece isground to a set size, and wherein in response to said sizing signal fromsaid sizing device, the infeed movement of a wheel head rotatablycarrying a grinding wheel is halted in a rough grinding operation priorto a fine grinding operation, the improvement comprising:size settingmeans for storing a workpiece diameter upon which said sizing signal isto be issued; infeed halt means for halting the infeed movement of saidwheel head upon the issuance of the sizing signal; diameter read commandmeans for outputting a diameter read command to said sizing device inorder to cause said sizing device to measure the actual diameter of saidworkpiece when a grinding caused by a spring-back motion of saidworkpiece ends after the halt of said infeed movement; reference valuestoring means for storing a reference value which indicates a diametersize of said workpiece having a desired allowance on account of saidsubsequent fine grinding operation; difference value calculation meansfor calculating a difference between said reference value and saidactual workpiece diameter measured in response to said diameter readcommand; and sizing compensation means for compensating said workpiecediameter stored in said size setting means for said differencecalculated by said difference value calculation means.
 2. A grindingmachine according to claim 1, wherein said diameter read command meanscomprises:awaiting means for awaiting for a predetermined time longerthan a time needed to end said grinding caused by said spring-backmotion; and command output means for outputting said diameter readcommand when said predetermined time has passed.
 3. A grinding machineaccording to claim 1, wherein said diameter read command meanscomprises:command output means for outputting said diameter read commandat a predetermined time interval after the halt of said infeed movement;and detecting means for detecting the variation velocity of the diameterof said workpiece at said time interval after the halt of said infeedmovement so as to stop outputting of said diameter read command whensaid variation velocity becomes a predetermined value, whereby adiameter of said workpiece measured in response to the last one of saiddiameter read commands from said command output means is treated as saidactual diameter of said workpiece.